Method for diagnosing performance problems in cabling

ABSTRACT

The present invention provides methods for using time domain analysis of NEXT, Return Loss and the like, in conjunction with the application of time or distance referenced limits to verify and determine compliance of the performance requirements of connections in a typical link. Time domain analysis of NEXT, Return Loss data and the like, suitably provides the performance characteristics of a link as a function of time or distance. When coupled with time or distance performance curves for connections, it can be determined if the transmission fault is at a connection or in the cable. The time limit curves for connections can be generated based on the frequency domain performance requirements for connecting hardware of a specific level of performance. The connection time limit curves thus provide an interpretation means to determine if the connection is within performance standards, allowing improved isolation of the fault condition.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional ApplicationSerial Number 60/133,932 filed May 13, 1999 has been expired.

BACKGROUND OF INVENTION

1. Technical Field of the Invention

This invention relates generally to the use of signal processingtechniques for determining the performance of cabling connections, and,more particularly, to the development and use of time domain limits todetermine the location and source of faults in cabling systems.

2. Background of the Invention

The transmission performance characteristics of modem high speed datacommunication twisted pair cabling systems are defined by variousinternational and industry working bodies (standards organizations) toassure standard data communication protocols can successfully betransmitted across the transmission media. These data communicationcabling systems (known as links) typically consist of connectors(modular 8 plugs and jacks) and some form of twisted pair cabling. Therequirements for important RF transmission performance parameters suchas, among others, Near End Crosstalk (NEXT), Return Loss, InsertionLoss, and Equal Level Far End Crosstalk (ELFEXT) are specified as afunction of frequency. To assure compliance of cabling systems withthese requirements, various field test instruments are available tocertify that installed cabling meets the required frequency domainlimits. These instruments perform various measurements to verifycompliance with the standards and provide an overall Pass or Failindication of the link.

When failures are detected in a link, a trouble-shooting process must bedone to make the link compliant with the requirement. However, currentlyknown field test instruments generally do not provide simplifieddiagnostic information to help locate and determine the reason forfailures. Determining the cause of some RF transmission parameter faultscan be difficult since the overall link performance often depends on theperformance of the individual components and installation techniques ofthe link. Until now, there have been few simple yet accurate methods todetermine if a fault is at the connection or the within the cableitself. That is, the frequency domain data that is processed bycurrently known field test instruments does not provide readilyinterpretable information about the cause of the failure.

In the past, trouble-shooting of NEXT and Return Loss faults with normalfield test equipment has generally been done on a trial and error basisas it typically requires skill levels normally not available to thecable installation industry. Often, misinterpretations of frequencydomain data are made, resulting in substantial unnecessary rework of thelink.

For example, transmission parameter requirements have been establishedfor various classes or categories of performance for structured datacommunication wiring systems. There are current standards for Category3, (10 Megabit/second data systems), Category 5 (100 Megabit/second datasystems) and new emerging standards (Category 6 and Category 7) tosupport even higher data rate systems. The Category 5 cabling system isa mature technology and few installation problems exist due to theexcess margin that has evolved in the individual component designs.

However, with the emergence of higher performance Category 6 andCategory 7 cabling systems, a significant percentage of such links donot meet desired performance levels. These links thus require a faultdiagnosis. In general, the failures in these links are due to a lack oftransmission performance margin in individual components and higherdegrees of sensitivity to the installation practices that have been usedfor Category 5 and other systems that are pervasive in the market.

For example, a typical link 100 in a structured cabling system andassociated field test configuration is shown in the FIG. 1. Link 100consists of a data communication patch panel 110 (for example, locatedin a wiring closet), four-pair twisted pair cable 120, and a dataconnector 130 in a work area. Field testing of the link transmissionperformance is typically done with field test equipment 140 that runs asuite of frequency domain tests from both ends 122 of link 100. Thefield test equipment is interfaced through short test lead cables 150that connect to data communication jacks 110, 130 of the link undertest. Tests of NEXT, Return Loss, Insertion Loss, ELFEXT and the like,are typical measurements performed by these instruments to certifytransmission parameters. The measurements are then compared to a set ofknown limit criteria established for specific categories of performance.A Pass/Fail indication is then made.

An example of a NEXT measurement and the performance limit for aCategory 6 link 100 is shown FIG. 2. The measured performance of link100 exceeds the limit at one or more measured frequency points. The linkis considered to have failed because it does not meet desiredperformance standards. The data in FIG. 2 shows a failure was detectedat several regions of the frequency spectrum. A challenge in diagnosingthis failure is determining if the cause of the failure is theconnectors 110,130, cable 120, or the installation practices employed toterminate cable 120 to connectors 110,130. There is little informationin the frequency domain graph of the magnitude of NEXT to help with theproblem isolation process. Thus, a significant first step in thediagnostic process for the example shown in FIG. 2 is to locate theposition of major contributors of NEXT in link 100 in time, and hence,distance.

Those skilled in the art understand the conversion from the frequencydomain to the time domain may be accomplished by applying an InverseFourier Transform process to the magnitude and phase NEXT frequencydomain data. The result of this conversion provides a plot of changes inNEXT vs. time/distance. For example, the NEXT time response for thepreceding example is shown in FIG. 3. As seen in the graph, there are anumber of large sources of NEXT. The first major source is connector 110 located approximately two meters from the ‘near’ end 110 of testcable 120. As is apparent from the graph shown in FIG. 3, other largesource NEXT exist within cable 120 itself

Time domain techniques have been used to identify sources of NEXT incurrent field test equipment. However, knowledge of NEXT vs. timeinformation does not necessarily aid in diagnosing the reason forfailure. Time data itself can be useful since it identifies sources ofNEXT as a function of distance; but this data itself does not provideinformation as to whether connector 110 or cable 120 performance iswithin required performance ranges. In FIG. 3, conventional wisdompoints to connector 110 as the non-compliant component since it is thelargest source of NEXT. However, without additional data, there is nodefinite information as to how to resolve the failure.

One method of trouble-shooting the NEXT failure in link 100 is todisassemble link 100 and qualify the NEXT characteristic of eachcomponent relative to the component requirements. FIG. 4 shows NEXTmeasurement results for both cable 120 and connector 110 compared to oneanother and the respective NEXT limit for each component. In this case,connector 110, which was the highest NEXT source, falls withinacceptable NEXT limits, and thus its performance requirements. However,the graph shows NEXT in cable 120 exceeds acceptable NEXT limits forcable 120. The cause of this failure is cable 120 and not connector 110.Time constraints, the knowledge and experience of cable technicians, andthe impracticalities of reworking cabling systems make such disassemblyfor diagnosis impractical to do in the field.

Further, as FIG. 4 shows, the field diagnosis problem is quitedifficult. Frequency domain measurements of link 100 do not generallyprovide fault location information. While time domain techniques areuseful for locating sources of NEXT, they lack limit information todetermine the components that are non-compliant. Thus, combining the useof time domain measurements with a method to convert the frequencydomain component limits to the time domain would produce a majorenhancement to the field diagnostic capability.

Another possible method to calculate the time limit and display timedomain NEXT and Return Loss data is to attempt to normalize the limitdata for attenuation with distance. Generally, this method generates aflat time limit line and produces data that has a flatter response withdistance. Such a normalization process is disclosed in U.S. Pat. No.5,698,985, entitled “CrossTalk Measurement Instrument with SourceIndication as a Function of Distance,” issued to Bottman and assigned toFluke on Dec. 16, 1997 (the “'985 patent”). While this approach appearssimple and attractive, in practice it is difficult to properly implementand can provide misleading information. This is because the attenuationdue to cable 120 is both a function of frequency and distance.Generally, the attenuation increases at approximately the square of thefrequency. To normalize the time response for length generally requiresspecial processing with time dependent filters that account for thelength and transfer function. Simple scale factor normalization based onlength as described in the '985 patent tends to enhance the lowfrequency contribution of the data with increasing length, leading topossible misdiagnosis of fault conditions.

Thus far, the examples and discussion have been related to NEXTmeasurements. However, additionally, return loss measurements have manyof the same diagnostic issues that can be addressed in a similar mannerusing time domain limits and processing techniques.

Return loss measurements provide a measure of the ratio of the reflectedenergy to the transmitted energy. Generally, signal reflections occurwithin data communication cabling due to impedance changes in thetransmission media. Major sources of reflection can occur at connectionsdue to connecting hardware, poor installation, a change in cableimpedance and the like. The normal certification tests are done in thefrequency domain to verify the parameter is compliant with therequirement. The frequency domain Return Loss measurement provides anoverall measure of reflected energy. However, the measurement does notseparate the reflected signals from each of the individual components.

Accordingly, a method for determining if the transmission parameterfault is at the connection or in the cable would be of benefit to thecable installation industry. Since typical links are constructed of asingle connection at each end of the cable, isolating the problem toeither the connection or the cable allows for rapid rectification of theproblem. Further, the application of time domain techniques along withtime limits for the reflection that is allowed at a connection pointwould provide the installer with a means to diagnose Return Loss faults.Enhanced diagnostic capabilities are required to help successfullyinstall and certify new higher performance cabling systems tosignificantly improve the productivity of the installation process withsuch diagnostics.

SUMMARY OF THE INVENTION

The present invention provides methods for using time domain analysis ofNEXT, Return Loss and the like, in conjunction with the application oftime or distance referenced limits to verify and determine compliance ofthe performance requirements of connections in a typical link. Timedomain analysis of NEXT and Return Loss data. suitably provides theperformance characteristics of a link as a function of time or distance.When coupled with time or distance performance curves for connections,it can be determined if the transmission fault is at a connection, inthe cable, or in another component of the link. The time limit curvesfor connections can be generated based on the frequency domainperformance requirements for connecting hardware for a specific level ofperformance. The connection time limit curves thus provide aninterpretation means to determine if the connection is withinperformance standards, affecting improved isolation of the faultcondition.

BRIEF DESCRIPTION OF THE DRAWINGS

Additional aspects of the present invention will become evident uponreviewing the nonlimiting embodiments described in the specification andthe claims taken in conjunction with the accompanying figures, whereinlike numerals designate like elements, and:

FIG. 1 is a typical link and field test configuration;

FIG. 2 is a graph showing NEXT vs. Frequency for a failing Category 6Link;

FIG. 3 is a graph showing NEXT vs. Time;

FIG. 4 is a graph showing the NEXT of a cable and connector from afailing link;

FIG. 5 is a graph showing the link NEXT time response for a failing linkwith a connector time limit a non-compliant cable;

FIG. 6 is a graph showing the transformation of connector frequencydomain, characteristics to the time domain;

FIG. 7 is a diagram showing the process to calculate time limits;

FIG. 8 is a block diagram of a frequency domain cable fieldcertification tool with time domain diagnostics; and

FIG. 9 is a graph showing the incremental process of building a timelimit line in accordance with the present invention; and

FIG. 10 is a flow chart of steps in accordance with an illustrativeembodiment of the present invention.

DETAILED DESCRIPTION OF PREFERRED EXEMPLARY EMBODIMENTS

The following descriptions are of exemplary embodiments only, and arenot intended to limit the scope, applicability, or configuration of theinvention in any way. Rather, the following description provides aconvenient illustration for implementing a preferred embodiment of theinvention. It should be apparent that various changes may be made in thefunction and arrangement of the elements described in the followingembodiments without departing from the spirit and scope of the inventionas set forth in the appended claims.

That being said, with momentary reference to FIG. 10, a flow chartshowing a process in accordance with the present invention isillustrated. First, a performance limit for a cable is determined 1000.Next, a location variable on the cable is set to an initial referencepoint 1010 and a performance characteristic at the location variable isobtained 1020. This process is suitably repeated for the length of thecable. That is, after measuring the performance characteristic at thelocation variable, the location variable is moved an incremental amountdown the length of the cable to reset the location variable and theperformance characteristic is measured at the new point 1030.

Accordingly, the present invention relates generally to the developmentof a means to convert frequency domain limits for cabling linkcomponents into corresponding time domain limits and then to apply theselimits to a diagnostic process to determine the location of atransmission fault.

It is generally recognized that LAN connecting hardware components areelectrically short, and may have NEXT and Return Loss characteristicsthat may generally be considered as point NEXT and reflection sources.Ideal characteristics of these point sources can be described by simplefrequency domain equations and converted to the time domain via varioussignal processing techniques such as Inverse Fourier Transform signalprocessing. Performance time limits are then used to determine if aconnection meets the requirements of a certain category of performance.

An exemplary embodiment of applying a component time limit to thediagnostics problem in accordance with the present invention is shown inFIG. 5. In this graph, the time domain limit for a Category 6 connectoris superimposed on the time data from the preceding failing Category 6link example. Alternatively, other types of connectors may also betested utilizing the methods and apparatus of the present invention. Thetime limit is denoted as an S-Band™ (Standard Connector Band). Acompliant connection falls within these limits. Thus, FIG. 5 shows theconnection is compliant with the NEXT specification for connectinghardware. Since the connector and its installation are within desiredspecifications, it can be concluded that the cable is the non-compliantcomponent. The combination of time domain measurements to separate andlocate sources of NEXT and a method to derive and apply time limits forconnectors based on the frequency domain requirements thus solves adifficult diagnostic problem in this link.

In accordance with an exemplary embodiment of the present invention,FIGS. 6a,b show a transformation of an ideal NEXT response 600 a to thetime domain. The computed time domain response for this transferfunction is an ideal band limited impulse response 600 b located at zerodistance from the signal or test source. The transformation processallows computation of impulse response 600b for ideal NEXT sources,Return Loss (i.e., reflection) sources or the like whose frequencydomain characteristics suitably follow the requirements for specificcategories of performance for cables such as Category 5, 5E, 6 and 7 andthe like. A peak response 602 of the limit impulse represents anallowable maximum for a measured time response in order to be compliantwith the frequency domain limit 600 a.

In a preferred embodiment of the present invention, the preceding limittransformation suitably allows calculation of the time response limitfor a single coupling or reflection source located at zero distance fromthe test source, though the connectors in a link may be located atvarying distances from the test source. For example, the distance to theconnector is typically about 1 to 2 meters (the length of the testcable). The time limit thus would be calculated for about two meters orgreater. A series of responses is then suitably calculated atincremental distances beyond zero distance from the test source toderive a smooth limit line that extends beyond the expected position ofthe first connector. The computation of this time limit line thussuitably accounts for both propagation delay and frequency dependentattenuation of the cable that connects the test source to the firstconnection of the link.

The determination of the time limit is generally done by any suitablemeans. However, a method for obtaining the entire time NEXT limit inaccordance with a preferred embodiment of the present invention isoutlined in FIG. 7. First, with reference to FIG. 7a, the frequencydomain connector limit is used as its transfer function. Then, assumingnominal frequency dependent attenuation characteristics of cable 120,FIG. 7b illustrates the computation of the overall frequency domaintransfer function when the connector is placed at increasing incrementaldistances down the cable so that impulse response 600 b for the transferfunction at each distance is processed. Next, as shown in FIG. 7c andFIG. 9, the envelope of the peak responses 700 for all distances isdetermined to compute the time domain limit for the connector.

In accordance with an alternate aspect of the present invention, analternative method to obtain the desired connection limit curve is touse a connection of suitable characteristics and physically position theconnector at incrementally increasing distance from the reference pointand measure the time response of the connector at each distance. Thelimit curve can then be represented by a smooth curve that correspondsto the peak time response of the connection at each distance.

In accordance with the present exemplary embodiment, the foregoingmethod for calculating a NEXT time limit begins with assuming connectorNEXT characteristics that generally follow defined frequency limits forspecific categories of performance (e.g., as Category 5, 6, 7 and thelike). That is, the connector is assumed to be located at time zero orzero distance from the test instrument. The overall NEXT characteristicsare then calculated and transformed to the time domain via suitableprocessing techniques such as an Inverse Fourier Transform, WaveletTransform, Hartley Transform or the like. The connection is then assumedto be located at small incremental distances from the test instrument,and the overall NEXT characteristics are then calculated assumingsuitable cable attenuation and propagation constants. This response isthen transformed to the time domain. The process is repeated assumingthe connection is moved small incremental distances from the testinstrument and the envelope of the peak time responses for each distanceis calculated using a suitable means to form a smooth time limit line.Impulse frequency response 600 b is then applied to limits 700 and thelocation of any failure in the link is determined by observation of thelocation where response 600 b exceeds limits 700.

According to an alternative embodiment of the present invention, asimilar method can be used to calculate Return Loss time limits.Generally, in the case of Return Loss, impedance differences between thetest cable and link cable are considered along with the connectorreflection characteristics. A process to generate the Return Loss timeassumes a minimally compliant connector and compliant impedances of thetest and link cable. Similar to the embodiment described above withrespect to determination of NEXT faults, a frequency domain return lossfor these elements is computed with the connection at increasingdistances from the source. The time response is calculated at eachdistance and the envelope of the peak responses is again formed as anallowable time limit. Again, the resulting time limits are a set ofsubstantially symmetric curves that may be normalized to unity at timezero.

In accordance with an additional aspect of the present invention and inthe preferred embodiment, normalization to unity at time zero makescomparisons to actual data more convenient to interpret. The time limitcurves decrease in magnitude to follow the nominal frequency dependentattenuation that is expected in the cable. NEXT and Return Loss timedata suitably reduce at this rate due to round trip attenuation of thecable. NEXT and Return Loss time limits can be calculated for connectorsof any performance level. For example, for field diagnostic purposes,particular time limits are calculated for Category 5, 5E, 6, and 7connectors.

A simplified block diagram of a preferred embodiment of a cable fieldcertification instrument 200 with enhanced diagnostic capabilities isshown in FIG. 8. The measurement system 200 is based on a sweptfrequency domain system that detects both magnitude and phase of thereceived signals. Full vector frequency domain data is suitably used toallow time information to be extracted by, for example, a Fouriertransform signal processing technique. The breadth of measurement rangeis generally known as the “dynamic range,” and test instrument 200 isable to measure signals having a range of about 100 dB. The frequencydomain approach of the present invention suitably supports such adynamic range due to its ability to generate large signal powers at asingle frequency. The large transmit signal thus maintains the smallerreceived signal above intrinsic circuit noise levels.

In accordance with the presently described exemplary embodiment, testinstrument 200 measurement circuits consist of a frequency generator 202of about 1 MHz-750 MHz, a switch matrix 204 to connect a transmitter 206which transmits the test signal, and a receiver 208 which receives theresponse signal from the link being tested, and signal processingcircuits 215 for detection and limit checking of received data using themethods of the present invention. Preferably, the receiver function isimplemented through narrow band heterodyne techniques that allowtracking of the transmit signal and rejection of undesired signals.

Preferably signal processing circuits 215 of the present exemplaryembodiment consist of 217 a suitable analog to digital converter, adigital filter and detector 218 which processes one or more digitallyconverted samples of the intermediate frequency output of the mixer 222into a complex i.e. real and imaginary value for each frequency of test.These complex valued signals are mathematically converted into magnitudeand phase using a:suitable rectangular-to-polar transformation mechanism219. These frequency domain data results may also be processed andstored in internal or external memory for further certification tests.The magnitude values are further suitably converted into decibel valuesand compared frequency-by-frequency to the suitable frequency domainlimits 221, and a pass or fail indication is suitably generated and sentto the control processor 212. The complex valued signals from digitalfilter and detector 218 are also routed to a suitable filter/windowingfunction 214 in order to provide a desired level of anti-aliasing andeffective impulse shaping, described in more detail below. Filter/window214 applies a suitable scaling coefficient to each complex data value(sample) according to the frequency at which it was acquired. Thesewindowed complex signals for each frequency are taken together andapplied to an inverse Fourier Transform mechanism 220 (for example, aCooley-Tukey Fast Fourier Transform or the like), to suitably convertthe frequency domain signal into a time domain representation.

According to the various aspects of the present invention, controlprocessor 212 initiates tests in response to user commands through aninput device 210 such as, for example, a keyboard, keypad, push buttonsor the like. The test results are suitably stored in a standard controlprocessor memory. The native frequency domain data results can beprocessed and stored for any certification tests. Link specificationlimits may be defined as a function of frequency and reports may be usedto show at what frequency failures occurred. In accordance with analternative aspect of the present invention, a similar measurement andprocessing system may be used in the remote scanner unit to allow thesame measurements to be performed at the far end of the link.

In accordance with the present invention, the user initiates diagnosticmeasurements via keyboard 210 commands or through other suitable meansif a failure is detected. In the preferred embodiment, these commandsinitiate a NEXT or Return Loss measurement on the desired pairs ofcables up to the full frequency range of about 1-750 MHz. The frequencydata may then be processed and detected and passed through a frequencyfilter and window prior to the Inverse Fourier Transform operation.These measurements can be done at either end of the link in a main orremote unit and to allow diagnostic functions to be performed on eitherend.

In accordance with another aspect of the present invention,filter/window function 214 is applied to the data prior to the inverseFourier operation. The window function 214 is suitably selected toproduce substantially flat time filters and band limit data within thecomponent specified frequency range. Preferably, filter/window function214 is determined by the category of link that is to be tested. Forexample, Category 5 and 5E component performance might be specified asabout 100 MHz, and therefore an appropriate filter/window is selected tolimit the high frequency data that is transformed to the time domain. Incontrast, for Category 6 and 7 links, the window function wouldpreferably limit data to approximately the 250 MHz maximum frequencyspecification for Category 6 and 7 links.

Next, the filtered data is suitably processed via a 1024 point inverseFFT algorithm to convert it to the time domain where it may then bedisplayed on a graphic display module 216 of the instrument. The timedata is suitably scaled relative to the selected time limit which mayalso be displayed for diagnostic reference. The time limit informationmay be pre-computed and stored in the processor control memory. Theappropriate time limit curve may be displayed as a function of theperformance of the category of the tested link. In the preferredembodiment, the x-axis of the graph is scaled in length to allow theuser to determine the distance to a fault or event. The user can thenview the time response to determine if the link connections are withinthe time domain limits and make a decision as to whether the connectionis the cause of the fault. Thus, the combination of the time limits andtime domain presentation of NEXT and Return Loss data in datacommunication cable field test equipment provides a significantenhancement to the field trouble shooting process.

Lastly, it should be apparent that while the principles of the inventionhave been described in illustrative embodiments, many combination andmodification of the above-described steps, structures, arrangements,proportions, the elements, materials and components, used in thepractice of the invention in addition to those not specificallydescribed may be varied and particularly adapted for specificenvironments and operating requirements, without departing from theprinciples of the present invention. For example, though notspecifically described, it is possible to make NEXT and Return Lossmeasurements via a time domain pulse transmission approach. Theseresults can then be displayed relative to a connector time domain limitto provide the enhancement to the diagnostics function and still fallwith the ambit of the appended claims.

We claim:
 1. A method for determining limits for use in cabling diagnostics, comprising: (a) determining desired performance limits for a component in a reference cable link; (b) setting a location variable to said component to an initial reference point; (c) obtaining a performance characteristic at said location variable; (d) moving said location variable an incremental amount to reset said location variable; and (e) repeating steps (c)-(d) along the length of said reference cable link until a desired limit is obtained.
 2. The method according to claim 1 wherein said location variable is at least one of a spatial variable or a temporal variable.
 3. The method according to claim 1 further comprising the step (e) of comparing said performance characteristics of said component to said desired performance limits and outputting a result of said comparison.
 4. The method according to claim 1 further comprising the step (e) of determining whether said performance characteristic exceeds said performance limits.
 5. The method according to claim 1 wherein said obtaining step further comprises calculating a time domain performance characteristic by measuring said performance characteristic at said reference point.
 6. The method according to claim 1 wherein said obtaining step further comprises calculating a time domain performance characteristic by mathematically calculating said performance characteristic at said reference point.
 7. The method according to claim 1 wherein said obtaining step further comprises calculating a time domain performance characteristic by simulating said performance characteristic at said reference point.
 8. The method according to claim 1 wherein said obtaining step further comprises generating a time domain performance characteristic by performing an Inverse Fourier Transformation on said performance characteristic at said reference point.
 9. The method according to claim 1 wherein said performance characteristic is a reflection characteristic reading.
 10. The method according to claim 9 wherein said reflection characteristic reading is a return loss reading.
 11. The method according to claim 1 wherein said performance characteristic is a signal coupling characteristic.
 12. The method according to claim 11 wherein said signal coupling characteristic is a cross-talk reading.
 13. The method according to claim 12 wherein said cross-talk reading characteristic is a near end cross-talk reading.
 14. A method for determining limits used in diagnosing cabling, comprising: (a) determining desired performance limits for a component in a reference cable link; (b) setting a location variable to a predetermined distance from a reference point; (c) calculating a frequency performance characteristic at said location variable; (d) transforming said frequency performance characteristic into a time domain performance characteristic; (e) adding an incremental amount to said location variable to reset said location variable; and (f) repeating steps (c)-(e) along the length of said reference cable link.
 15. The method according to claim 14 wherein said location variable is at least one of a spatial variable and a temporal variable.
 16. The method according to claim 14 further comprising the step (g) of comparing said time domain performance characteristics to said desired performance limits.
 17. The method according to claim 14 further comprising the step (g) of determining whether said time domain performance characteristic exceeds said desired performance limits.
 18. The method according to claim 14 wherein said obtaining step further comprises calculating a time domain performance characteristic by measuring said performance characteristic at said reference point.
 19. The method according to claim 14 wherein said obtaining step further comprises calculating a time domain performance characteristic by mathematically calculating said performance characteristic at said reference point.
 20. The method according to claim 14 wherein said obtaining step further comprises calculating a time domain performance characteristic by simulating said performance characteristic at said reference point.
 21. The method according to claim 14 wherein said obtaining step further comprises generating a time domain performance characteristic by performing an Inverse Fourier Transformation on said performance characteristic at said reference point.
 22. The method according to claim 14 wherein said performance characteristic is a signal coupling characteristic.
 23. The method according to claim 14 wherein said performance characteristic is a reflection characteristic reading.
 24. The method according to claim 14 wherein said reflection characteristic reading is a return loss reading.
 25. The method according to claim 22 wherein said signal coupling characteristic is a cross-talk reading.
 26. The method according to claim 25 wherein said cross-talk characteristic is a near end cross-talk reading. 